South East Rivers Trust (& the Wandle Trust)

Troubleshooting on the River Darent

Otford to Park Farm Fishery Walkover – Spring 2019

The River Darent rises from sandstone springs around Sevenoaks, Kent and to the west around Westerham. Flowing north, further chalk springs also contribute to the river as it makes its way towards Dartford, where it discharges into the Thames estuary. 

The River Darent (and the neighbouring River Cray) are chalk streams, which are unique, globally-rare, freshwater habitats. There are only around 230 chalk streams in the world and we are lucky in the UK to be the custodians of about 80% of these. Chalk streams are fed by underground aquifers and as such are characterised by high water quality and, naturally, would maintain a strong base flow all year round. 

This high-quality water also serves as a valuable water resource for drinking water abstraction in the South East and has been used for this purpose – increasingly so, since the turn of the century. Due to over-abstraction, low flow issues on the Darent and Cray became evident during the drought of 1976 and subsequent droughts of the 1990s when the river dried up altogether. These incidents prompted local action with the ‘Darent Action Plan’ which reduced groundwater abstraction. However, the River Darent is still suffering from significantly reduced flows and is still at risk of drying up completely in equivalent drought events. 

We recently carried out a troubleshooting walkover from the bottom of Otford village allotments to the downstream extent of Park Farm Fishery, a relatively short distance of approximately 1.5km. The day was warm, sunny and dry, and the weather had been pretty much dry with minimal rainfall the previous week.

A number of issues were affecting the river through much of the reach: these are summarised with example photos below.

Geomorphological activity

The issues:

Throughout this reach, river gravels were old, sealed with sand and fine sediment and totally static.

Throughout the reach the river bed gravels were dark and discoloured, indicating a lack of geomorphological movement.

Although there is clearly erosion and deposition of fine sediment and sand through the reach, there were only two areas where the river gravels were clean and fresh. These were associated with a macrophyte bed (water crowfoot, aka Ranunculus in this case) and a fallen tree, which both constricted the flow, enabling ‘stream power’ energy to reach a level where gravel can be mobilised and ‘sorted’. Freshly moved loose gravels are lighter in colour than static gravels, which look dark in colour due to biofouling by diatoms and algae.

Ranunculus and fallen tree constricting the flow which has led to scouring and mobilisation of sand silt and gravels improving the quality of the substrate.

This lack of clean gravels may be due to:

  1. A lack of complex habitat ‘structure’ (e.g. woody debris, macrophyte beds etc.) in the river;
  2. A symptom of low flow due to abstraction pressure, with the existing channel being now too wide to generate the energy required in channel to move gravels;
  3. Gravels sealed with sand making them less likely to become mobile; or
  4. A combination of the above.

Mobile, loose, free moving gravels provide micro-habitats for invertebrates, allow rooted macrophytes (like Ranunculus) to establish and are essential for salmonid (trout) and rheophilic coarse fish spawning.

For more information on fish spawning please read our previous blogs:

The solutions:

Large Woody Material (LWM) / Flow Deflectors

Gravel movement and the displacement of accumulated silt and sand can be encouraged by the addition of upstream pointing flow deflectors.

Water velocities increase around the tip of the deflector, increasing localised stream power so that gravel, and the fine sediment accumulated within it, can be mobilised. Gravels will be deposited a short way downstream, providing a clean fresh pile of loose gravel for fish to spawn on.

The deflector then contributes to optimal conditions for incubating fish eggs that could be laid in these clean gravels. Silt and sand will be transported further downstream due to the smaller grain size and mass, and deposited in a low velocity zone.

Positioning deflectors at a 45-degree angle to the channel facing upstream in pairs (one on each bank) towards the head of gravel riffles will bring the most benefit to trout spawning habitat and will prevent bank erosion.

Reduce shading

Reducing riparian tree shading (see section below) will encourage rooted macrophytes (including Ranunculus) growth, which will provide a wide diversity of flow patterns including high velocity flows where the flow is constrained between macrophyte beds, increasing localised stream power which will help move and clean gravels (as described above).

Gravel augmentation

Rehabilitating existing gravels in situ using the methods described above may take time and will require high flow flood events to provide the energy required to move significant quantities of gravel. Augmenting existing riffles, or building new riffles in over deep / wide areas (where the river bed is significantly covered in sand / silts), with fresh clean gravel could provide a parallel solution and accelerate the process and provide additional spawning habitats. New riffles can be built using a foundation of larger materials, grading up to cobbles and gravels at the top.

Channel narrowing

Similarly narrowing the channel at riffle sites can increase stream power over the riffle and increase the mobility of gravel, silt and sand. The over wide channels could be narrowed by establishing a new bank line with staked in faggot bundles and back filling with brash. The new bank line should be established on the inside of bends and follow a meandering form. The width of the new channel should be determined following study of nearby functional / geomorphologically active sections.

Channel narrowing can also be used in combination with gravel augmentation to build new riffles in existing over deep / wide areas. Existing riffles along this stretch of the Darent are not mobile and do not provide clean spawning gravels, so channel width dimensions should be reduced to promote this.

Gravel raking, forking, cleaning

Riffles can also be raked, forked or cleaned with high pressure water jets or leaf blowers, to clear them of fine sediments and break up their sealed nature. Breaking gravels up with a strong garden fork prior to jetting with a leaf blower is probably the most effective and efficient method.

Native brown trout will spawn from the end of November to January, so riffles cleaning just prior to this period is likely to bring the most benefits to spawning habitats. The objective of the cleaning should be to free up, loosen and remove the fine sediments from the gravels to a depth of approximately 6 inches. Specific areas can be targeted where fish are most likely to spawn, such as the heads of riffles, areas where the channel and thus river’s flow is constricted (e.g. off the ends of in-channel structures like flow deflectors, fallen branches etc.). 

Over-shading by riparian trees

The issues:

This is a common issue throughout the River Darent, where bankside trees have gone un-managed for a number of years, thus over-shading the channel.

The River Darent channel below Otford village, a tunnel of riparian tree cover over-shading the channel. Even when not in leaf, the river channel is shaded by this un-managed vegetation.

During the walkover, it was apparent that the only macrophyte beds in evidence were where tree cover was thin and direct sunlight was able to reach the river bed. Macrophytes are plants, so they require sunlight for photosynthesis, to grow and provide habitats for invertebrates and fish.

The diversity of flow caused by macrophyte beds provides numerous ecological niches for fish and invertebrates and encourages a biodiverse river community. Macrophytes also ‘slow the flow’, holding water back in the river, increasing depths and providing drought resilience.

Ranunculus in direct sunlight.

Similarly, over-shading can prevent the vegetation of natural berms. The reach contained numerous natural berms where sands and silts were deposited in low velocity areas, usually on the insides of bends. These are part of the natural processes of erosion and deposition that define river channels.

Natural berm formation on the inside of a bend where low velocities promote the deposition of sands and silts.

Un-vegetated berms can be vulnerable to becoming mobilised and eroded during high flow flood events. Vegetation of berms makes them more stable and helps consolidate and bind them together, reducing this risk.

Left: Dappled shade inhibiting vegetation of a natural berm. Right: Vegetation of a natural berm, allowed by expose to sunlight.

The solutions:

Tree management: selective felling and pollarding

Areas of shade are important to provide diversity and to help regulate water temperatures during high summer. Selective tree works should be targeted around shallow riffles, where Ranunculus is more likely to establish and where water speeds are higher, so exposure to solar radiation is minimised. The aim should be a diverse mosaic of shaded and un-shaded habitats of different types.

Weirs causing impoundments and barriers to fish migration

The issues:

In-channel structures like weirs and sluices present barriers to fish migration, degrade upstream habitats through the impoundment and prevent geomorphological processes such as erosion, deposition and sediment transport. These processes create the habitats that wildlife has evolved to utilise. As such the removal of redundant barriers is supported by a large weight of scientific evidence and is often the first step in any river restoration scheme.

For more information read our previous blog on this subject:

Three such barriers were identified during this walk:

  1. Otford Mill (just upstream of the reach walked);
  2. Rock weir at the top of Park Farm Fishery;
  3. Bridge weir towards the downstream extent of the Park Farm Fishery.
Rock weir at the top of Park Farm Fishery.
Bridge weir towards the downstream extent of Park Farm Fishery.

The solutions:

Barrier removal

Both of the barriers pictured above will be partially passable to at least upstream moving adult native brown trout during certain flow conditions, but will present total barriers to most coarse fish species.

Fish passage issues aside, given the poor quality of trout spawning habitats due to the lack of geomorphological processes, the bigger issue here presented by these structures is habitat degradation. The upstream impoundments of both barriers are relatively short, being only 10s of metres, so the improvements gained by rehabilitating the pond like habitats created within the upstream impoundments themselves wouldn’t be huge. However, the effect the structures have on stream power through the impounded reach and beyond and the effect they have on sediment (gravel) transport is the bigger issue, given the observations discussed above.

Weirs inhibit natural processes by removing the river’s energy upstream of the weir, and releasing it in one spot below the weir. This inhibits natural geomorphological processes and prevents sediment transport, and can lead to an incised channel as sediment is eroded below the weir, but not replaced by sediment transport from above.

Under natural conditions, gravels move down a river, much like the water, only more slowly!

Sediment transported from upstream to downstream builds habitat features like riffles and makes such features dynamic, on the move, ever changing, with clean fresh gravels flushed clean of silt and sand. Removing both of these structures would bring much needed energy back to the river channels upstream, and help kick start natural gravel movements, that support good spawning habitats for trout and other fish.

Uniform habitat

The issues:

There was little diversity in bed material, partly due to the lack of geomorphological activity and the blinding of river bed substrates with sand.

This limits the availability of micro-habitats for all life stages of fish as well as the availability of feeding lies (spots where the fish wait for food to pass by) for adult trout. Trout are territorial and will seek out the most productive feeding lie they have access to and will defend these optimal habitats from other fishes.

Thus, a uniform flat river bed habitats can only support low densities of trout, as fish in these habitats are able to see each other and therefore move apart, resulting in low numbers of fish in any given area. Conversely, more varied diverse habitats can support higher densities, as the varied habitat prevents the fish from seeing their neighbours and thus chasing them away, so a given area can support a high number.

The solutions:

Promotion of geomorphological processes

Everything discussed above from barrier removal to tree management, through to gravel augmentation and flow deflector installation will contribute to kick-starting geomorphological processes which will build more diverse dynamic habitats.    

Introduction of cobbles 

Introducing larger cobbles (around 6 inches in diameter) or flints (a natural feature of chalk landscapes) has been demonstrated to improve the carrying capacity of riverine habitats for juvenile salmonids: see the graph below from Gardner & Ellis (2014).

Results of catch per unit effort (CPUE) electric fishing surveys demonstrating the benefit of enhancing habitats with cobbles on the River Wylye, Wiltshire. Control site vs enhanced site, letters above each column denote salmonid density classifications (A= excellent, B= good, C= fair, D= poor, E= absent) from Crozier & Kennedy (1994).

Over-wide channel

The issues:

As discussed above, the river channel in places is over wide, which contributes to the reduced geomorphological processes discussed at length above. Natural processes have led to berm formation which is narrowing the channel naturally in places. Elsewhere, the over wide channel has led to the deposition of sands and silt across the entire width of the riverbed, degrading the habitat.

Over-wide channel, where the river bed is totally covered with more than half a metre’s depth of sand and silt.

The solutions:

Channel narrowing and gravel augmentation

More information on both these solutions can be found earlier in this blog.

Other points of interest – lateral habitats

Two tributaries joined the main river through the reach; a minor tributary joining near the upstream Park Farm Fishery boundary and the Twitton Brook which joins a little way downstream. Both appear to be spring fed as the gravels are calcified, suggesting a groundwater source.

Minor tributaries provide valuable spawning and nursery habitats for trout, and refuge areas during times of high flow or poor water quality in the main channel (e.g. pollution incidents).

There is a deep scour pot below the confluence of the main river and Twitton Brook. This suggesting the Brook carries elevated flows at some times. Tree roots form a small lip but should be passable by juvenile and adult trout.

Top: Minor tributary joining near the upstream Park Farm Fishery boundary. Bottom: Twitton Brook confluence.